A variable gain amplifier is a device for maintaining a desired level of output signals by adjusting gain, in a radio frequency communication system.
FIG. 1a shows a circuit diagram of a conventional feedback type variable gain amplifier disclosed in U.S. Pat. No. 6,285,257.
The conventional feedback type variable gain amplifier disclosed in U.S. Pat. No. 6,285,257 is provided with an amplifier AMP1, and gain control circuit GC1 which is a feedback circuit coupled to the amplifier AMP1 and controls a value of the gain. The amplifier AMP1 includes a series-connected high frequency cutoff inductor L11 and amplifying field effect transistor FET11 across a power-supply potential terminal VDD and a ground potential terminal GND. The gate of the transistor FET11 is connected to a signal input terminal VIN, and the drain is connected to a signal output terminal VOUT. The gain control circuit GC1 includes a series-connected gain control field effect transistor FET12 and direct-current cutoff capacitor C11 across the signal output terminal VOUT and the signal input terminal VIN. Furthermore, a resistor R11 connected across the drain and the source of the gain control field effect transistor FET12.
In this conventional feedback type variable gain amplifier, the resistor R11 maintains levels at the drain and source of the transistor FET12 to the same potential, the capacitor C11 isolates levels at the gate of the transistor FET11 from the power-supply potential VDD. Furthermore, by varying a voltage Vgc applied to a gain control terminal Vc1, which is the gate of the transistor FET12, the transistor FET12 and the resistor R11 function as a variable resistor having a resistance value R(Vgc).
FIG. 1b shows a circuit diagram of an equivalent circuit of the gain control circuit GC1 in the conventional feedback type variable gain amplifier shown in FIG. 1a. The equivalent circuit of the gain control circuit GC1 is expressed by a series connection of a variable resistor R12 and a direct-current cutoff capacitor C11.
One can adjust the gain of the variable gain amplifier, shown in FIG. 1a and FIG. 1b, by varying the resistance value of the variable resistor R12 by the control voltage Vc1.
However, a noise figure of the variable gain amplifier is low owing to a parasitic capacitance of the transistor FET12. Moreover, there is a problem that an input impedance of the variable gain amplifier is varied according to the power levels of the input signal. Furthermore, low noise figure and diminution of the gain in a high frequency band make a range of frequency where the variable gain amplifier can be matched be narrow.